HR: 1340h
AN: T13C-1481    [Abstracts]
TI: Oxygen isotope constraints on the origin of island arc granitoids
AU: * Perez, R J
EM: perezrafa@gmail.com
AF: University of Puerto Rico, PO Box 9017, Mayaguez, PR 00681, Puerto Rico
AU: Cavosie, A J
EM: acavosie@uprm.edu
AF: University of Puerto Rico, PO Box 9017, Mayaguez, PR 00681, Puerto Rico
AU: Valley, J W
EM: valley@geology.wisc.edu
AF: University of Wisconsin, 1215 W Dayton St., Madison, WI 53706, United States
AB:
Granitic intrusions in island arcs constitute additions of juvenile crust from oceanic environs that ultimately get accreted to continents. The genesis of island arc granitoids is thus important to studies of the growth of oceanic and continental crust. Puerto Rico (USA) is a composite island arc terrane that preserves a record of plutonism from 85 Ma to 38 Ma (Cavosie et al., 2007 AGU). Mid-crustal granitoid plutons are exposed (~1 to 500 km²), but their origins are unknown, as no suspected parental magmas associated with the plutons (e.g., gabbro) are exposed. This study uses petrography, WR major elements, and oxygen isotopes of WR and zircon from granitoids and xenoliths to place better constraints on the origin of granitoid in the Greater Antilles island arc.
WR δ¹⁸O analyses were made with laser fluorination by IRMS at the Univ. of Wisconsin (uncertainties = 0.10 to 0.20‰, 2sd). The main plutons (Caguas, Rio Blanco, San Lorenzo, Utuado, Vieques) yield primitive δ¹⁸O(WR) values, ranging from 6.24 to 7.72‰ over a range of wt.% SiO₂= 58.03 to 66.54. Smaller stocks (<20 km²) yield higher δ¹⁸O(WR) values, ranging from 7.47 to 10.27‰. Qualitative petrographic analysis reveals that granitoids with δ¹⁸O(WR) >~7.5‰ are partially to pervasively altered.
Zircon preserves magmatic δ¹⁸O and is used here to quantitatively evaluate the measured δ¹⁸O(WR) values. If δ¹⁸O(Zrc) and wt.% SiO₂ are known, a comparison of measured vs. predicted δ¹⁸O(WR) can be made (Valley et al., 2005, CMP). The measured δ¹⁸O(WR) values record variable amounts of alteration, ranging from virtually undetectable, to WR δ¹⁸O elevations of ~4‰, indicative of low-T subsolidus alteration. The Δ¹⁸O (WR-Zrc) values using calculated δ¹⁸O(WR) yield the following fractionations: -1.57 to 1.00‰ for granodiorites (wt.% SiO₂=66 to 57); -0.85‰ for diorite (wt.% SiO₂=55); and -0.56‰ for the only gabbro analyzed (wt.% SiO₂=50).
Mafic xenoliths (53-57 wt.% SiO₂) from 4 granitoids yield Δ¹⁸O(WR granitoid –WR mafic xenolith) as follows: Rio Blanco (0.58‰), San Lorenzo (0.55‰), Utuado (0.42‰), and Vieques (0.06‰). Petrographic analysis of the xenoliths reveals igneous textures (e.g., oscillatory zoning in plag) and assemblages dominated by hbl + plag, indicative of an igneous origin in a hydrous melt. The xenolith results (with the exception of Vieques) are consistent with models for the derivation of the granitoid plutons from gabbroic parental melts represented by the xenoliths. The small fractionations of Δ¹⁸O(WR granitoid – WR mafic xenolith) are permissible for magmatic processes such as partial melting or fractional crystallization. Thus, samples of the elusive gabbroic source(s) for the granitoids in Puerto Rico have tentatively been identified.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8021 Melanges
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting